The invention relates to a magnetic revolution counter for unambiguous determination of a pre-specifiable number of revolutions to be determined for a rotating element that can be used advantageously in numerous engineering fields, especially in automotive engineering.
Sensors for determining an angular position according to various physical principles are widespread. It is common to them that the sensor signal is periodic after 360°, i.e. that the sensor cannot differentiate between 10° and 370°. Therefore, for tasks in which the angle must be determined beyond 360°, such as is the case e.g. with the steering wheel in an automobile, such sensors are combined with another sensor that is able to detect the number of revolutions. It is then possible to differentiate between 10° and 370° using a combination with a revolution counter. Solutions for determining the number of revolutions are known in which the number of revolutions (for instance between 1 . . . 5) can be suggested mechanically using the course of a spiral having N spiral arms. Other solutions use mechanical gears connected to two angle sensors. The angle can be determined, even e.g. from 0 to 5-360°, from knowing the structure of the gear and the angular position of two magnets that are connected to the various wheels of the gear. Common to all of these solutions is that they require a mechanical element, that is, they are not contactless and are therefore not wear-free. However, a contact-free solution is required for many applications, particularly in an automobile. This contact-free solution could be created in that the angular position is determined at every point in time (continuously) and in this manner it is possible to differentiate a transition of 359° to 360° from angle 0°. This requires that the sensor and an associated memory element be continuously supplied with energy. This conflicts with the automotive engineering requirement that determining the absolute angle in the range of for example, 0° to for instance 5-360° must be successful even if e.g. the onboard electronics are disconnected from the battery.
The Posital Company developed contact-free counting of the number of revolutions that satisfies these requirements in principle (Company announcement: “Kraftwerk im Encoder . . . ” [Power Source in the Encoder . . . ], www.posital.com). In this case, a Hall sensor is used for determining the angle (0-360°). The number of revolutions is measured there using a so-called Wiegand wire. This wire has special magnetic properties that ensure that after every revolution, due to the sudden movement of a magnetic domain wall through a wire that is a few millimeters long, there is a brief but sufficiently intense voltage pulse that can be inscribed in a FeRAM (ferroelectric random access memory), even without the FeRAM being connected to the battery. This solution thus satisfies the requirement for determining the number of revolutions in a wear-free and contact-free manner and also counts revolutions up to the maximum storage capacity of the FeRAM used without the current supply being applied. However, the automobile industry rejects this type of solution because cost-effective production and packaging is not possible due to the macroscopic size of the Wiegand wire and because there are problems with electromagnetic compatibility due to the high ohmic input of the FeRAMs.
Another sensor element for counting revolutions that satisfies the aforesaid requirements is known from EP 1 740 909 B1 (WO 2005/106395). This sensor element is in the shape of an extended spiral having N windings and comprises a stack of layers that has the “giant magneto-resistance effect” (GMR). The GMR stratified system in this sensor element essentially comprises a hard magnetic layer, which defines the reference direction, and a soft magnetic layer, these being separated by a non-magnetic intermediate layer. The outer rotating magnetic field to be detected is strong enough to change the magnetization direction of the soft magnetic layer due to the movement of the domain walls, but it is too weak to change the magnetization direction of the hard magnetic layer, which runs parallel to the straight sections of the extended spiral. The sensor element thus reacts to a rotating magnetic field with a change in resistance, whole and half revolutions being registered in the form of 2N+1 resistance values within the countable range of 0 to N revolutions. Each resistance value is therefore bijectively assigned to a half integer or whole integer revolution value. The magnetic structure remains unchanged if the magnetic field does not rotate. Given rotation, the magnetization directions change, regardless of whether the resistance value is read out or not. This means that the system even registers all changes in the rotating magnetic field when it has no current or power, and a current supply is needed only for read-out, that is, for determining the resistance.
One disadvantage of such an arrangement is that, due to the memory geometry used, each revolution requires a complete spiral winding, and the spiral must be very large geometrically when counting a large number of revolutions. This means that the probability increases that defects that occur during the manufacture of the spiral will lead to failure and thus to a reduction in the yield. In addition, the chip surface area increases in size, and thus the costs for such a sensor also increase. Moreover, when there is a large number of spiral windings, the design provided in EP 1 740 909 B1 automatically leads to problems in determining the number of revolutions. The usable swing in voltage, which initially results from one revolution, is scaled at 1/number of spiral windings. This swing in voltage is clearly too small for a reliable evaluation for N> to >>10. One alternative, which is provided in the aforesaid patent, does permit the full magneto-resistance swing at higher rates of revolution, but still has the disadvantage of a long spiral, and the advantage of the large swing is offset in that, instead of two electrical contacts, all spiral parts that form an unclosed circuit are provided with four electrical contacts and must be read out and processed electrically. When N=100, this is four hundred contacts, and thus the circuitry is very complex.
In addition to the attempts described in the foregoing to develop revolution counters based on moving domain walls, there are also proposals for creating a magnetic logic device using moving domain walls, even if these proposals are in a search field that is not as closely related to the invention. In this case, magnetic domains are moved by closed or branching magnetic strips such that logical functions such as AND, NOT, and XOR can be created, in addition to intersections and branches. Thus, in Science Vol. 296, 14 Jun. 2002, pages 2003-2006, Allwood et al. propose a sub-μm ferro-magnetic NOT gate and shift register in which the logical information can be inscribed in the logical construct using locally flowing currents and can be read out again only by means of a TMR effect. In this publication, domain walls are moved by rotating magnetic fields that act as the clock for the magnetic logic device. US patent 2007/0030718 A1 by these authors describes the use of such a magnetic logic device in which the magnetic domain walls are moved using electrical current pulses. The same authors provide another proposal for constructing a magnetic memory (US 2007/0047156 A1) using magnetic domains. For this, the identical loop-like structures described in the foregoing are placed above one another, with the same number of cusps that are required for attaining the maximum storage capacity sought in the foregoing. Each structure comprises a loop of a magnetic material that includes cusp-shaped protuberances that protrude into the interior of the loop. Only one of the cusps that projects into the interior is elongated and, in a special embodiment, has a geometry that differs in its width from the other cusps. The information is inscribed in the memory using this cusp by generating a magnetic domain. Each cusp has a branch that leads outward, and disposed on its end is a magneto-resistive element for reading out the information. A rotating magnetic field that the loops detect homogeneously acts as clock and power supply for transporting the domains and in a special embodiment acts as serial data channel. The basic principal of this arrangement is variable generation of domains whose number equals the information specifically to be stored.
The object of the present invention is to create a magnetic sensor system for a revolution counter, which magnetic sensor system permits any desired pre-specifiable number of revolutions to be determined, for instance up to values of N>4000 or pre-specifiably more, and that thus if desired goes far beyond previously known solutions and that at the same time enables a cost-effective and structurally small embodiment that does not suffer from the disadvantages of the prior art. In addition, the proposed solution should overcome significant disadvantages of prior proposed solutions, specifically the large number of contacts (see EP 1 740 909 B1) (at least 4·N, where N is the number of countable revolutions), which number thus increases linearly with the number of the desired count of revolutions, and thus when determining 256 revolutions has 1000 contacts, and the long length of the spiral (2·N·1) where 1 is the longitudinal extension (typically 200 μm) of the spiral, as depicted in the aforesaid EP document.